It is important to know how the degree of polarization (DOP) changes with differential group delay (DGD) when DOP is the feedback parameter for an adaptive polarization mode dispersion compensator (PMDC). We derived the mathematical expression of DOP for pulse sequence with arbitrary and Gauss waveforms under the condition of quasi-monochromatic wave, then analyzed theoretically the relation between DOP and DGD with different power splitting ratio and pulse width. Then we carried out the experiments of DOP with 10Gbit/s RZ/NRZ pseudo random sequence pulses and made comparison between theoretical calculation and experimental data. The comparison showed that they matched closely. We also verified by experiments that DOP is independent of fiber dispersion.